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Determining an Enthalpy Change of Reaction
Aim: to determine the enthalpy change for the displacement reaction:
By adding an excess of zinc powder it a measured amount of aqueous copper (II)
sulphate and measuring the temperature change over a period of time, you then
calculate the enthalpy change for the reaction.
Formula
This will give us heat which is opposite to enthalpy.
Diagram
25.5
00.00.00
TemperatureDisplay
Stopwatch
Zinc powder
Temperature
probe
Lid
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Data Collection
Time (s) Temperature (°C) ± 0.1 °C
Trial 1 Trial 2 Trial 3
0 22.8 22.4 22.5
30 22.8 22.4 22.6
60 22.8 22.4 22.5
90 22.8 22.5 22.5
120 22.8 22.5 22.5
150 39.8 40.8 51.2
180 55.3 67.9 69.8
210 65.5 70.2 69.5
240 69.4 69.0 67.9
270 68.2 67.3 66.5
300 66.6 65.9 65.1
330 65.3 64.5 63.7
360 63.9 63.2 62.5
390 62.7 61.9 61.3
420 61.5 60.7 60.0
450 60.4 59.5 58.9
480 59.2 58.5 57.8
510 58.2 57.4 56.8
540 57.4 56.3 56.0
570 56.4 55.3 55.1
600 55.6 54.4 54.4
630 54.9 53.5 53.4
660 54.2 52.7 52.7
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Qualitative Observations
690 53.6 51.9 52.0
720 53.0 51.1 51.3
This data was then
represented in 3 graphs,
1 for each trial. The
graphs showed the
change in temperature
over the time span. See
attached hand drawn
graphs.
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Before reaction takes placeThe copper sulphate, mostly made up
of water, is a clear bright blue.
During reaction
As the zinc is added to the copper
sulphate, the solution begins to heat up
until it reaches a maximum at which
point it begins to cool down
After reaction takes place
After the reaction is complete we are
left with the products, reddish copper
and white zinc sulphate.
Data Processing
Graphs
On the graphs the red line represents the slope at the time of cooling or the rate
of cooling, and the green line indicates where the reaction began. The point at
which the two lines meet is the theoretical maximum temperature for that trial.
(see graphs). This is the max temperature value we will use for our calculations.
Trial 1 2 3
Max
theoretical
Temperature
found from
graph (°C)
74.0 74.0 72.8
Calcualtions
average max temp = 73.6°C
As we have two of the
same values for our
temperature (74.0) the
last value may be an
outlier but there is no
way to be sure of this
with only three trials so
we will need to include it
in our calculations.
Here we are finding the
average of our maximum
temperatures, to have
one final, more accurate
value for our enthalpy
change formula.In the f ormula we
need the change in temperature
therefore we must calculate theaverage minimum temperature
of the three trials. This is just
the lowest temperature
recorded for each trial.
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average min temp = 22.6°C
= 51°C
Finding the Uncertainty
To find you subtract the
average minimum temperature
from the average maximum
temperature.
Then we use the heat equation
to find Q, assuming that the
copper sulphate is made up
completely of water and has a c
value of 4.18 KJKg-1
We used 25ml of copper
sulphate which equals 25g =
We want to find the heat
change per mole so we divide
by the number of moles
This is now the heat produce
per mole of copper sulphate
reacted.
We take the uncertainty of the
instrument and divide it by the
smallest measurement taken togive us the biggest uncertainty
then we add up the 2 (pipette
and temperature) and times by
100 to give us the percentage
uncertainty.
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Final answer
Q= ± 0.44KJmol-1
But because we are looking for enthalpy change we just have to switch the sign
as enthalpy change is opposite to heat change.
Final value for Enthalpy change
Conclusion
When we compare our value of approximately -213KJmol-1 it is very close to the
accepted value of -217KJmol-1. However it is not within the uncertainty. When I
did the percentage yield formula I found that I was less than 2% off (only -1.76%). There was bound to be a difference as experimental conditions are
never perfect. Also I did wonder if because we used the specific heat capacity of
water and not copper sulphate if it made a difference.
Evaluation
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Error Explanation Improvement
Heat loss
This was our major point of
error and probably the
main reason why our value
was below the literature
value. The calorimeter was
polystyrene but the top
would have let out a lot of
heat to the surroundings
that would not have shown
in our data as they would
not have warmed thetemperature probe
To improve this I would
proposed using 2 or maybe
3 calorimeters and placingthem inside each other and
then taping them together
airtight. This would add
insulation but also create
airspace, which would
insulate well. Then I would
cut out a polystyrene top
with a much smaller hole so
that the temperature probe
can fit more snuggly. This
should minimize heat loss
significantly and hopefully
give a more accurate value
for ∆H.
C value of water or
copper sulphate
I felt that by using the
specific heat capacity of
water for our calculations
we decreased the accuracy
of our result, as we did not
know the specific heat
capacity of copper sulphate.There is no way to know if
this made our value smaller
or larger.
The SHC of copper sulphate
is 4.184KJKG-1K-1. Obviously
this does not seem like it
would make a big difference
but I recommend that it wasused in future calculations
Stirring the zinc so
it reacts fully
When the zinc is added it
often gets stuck on the sides
and does not all react. It is
in excess but it may still
have affected our results
If this experiment were re
done I would recommend
larger quantities of both
copper sulphate and zinc to
make sure that they can be
mixed properly although
you will need to make sure
you are careful not to spill
them over the side.